1. Field of the Invention
This invention relates to a user grouping method using inter-cell interference coordination (ICIC) in mobile telecommunication using the pre-defined scales for information handover and measuring data established in a mobile telecommunication system. This method groups the cell users into center cell users and edge cell users to help employ inter-cell interference coordination (ICIC) and is a simple while efficient user grouping method.
2. Descriptions of the Related Art
The Orthogonal Frequency Division Multiple Access (OFDMA) technology is the important technology to execute the 4th generation (4G) mobile communication technology. IMT-Advanced/4G technologies developed by the primary standards organizations, such as IEEE 802.16m, 3GPP LTE-Advanced, and 3GPP2 UMB+, are all air interface technologies based on the OFDMA technology.
In the OFDMA system, the time-frequency two-dimensional electric waves are composed of an Orthogonal Frequency Division Multiplex (OFDM) symbol in the time domain and a frequency subchannel in the frequency domain. Each frequency subchannel is composed of a plurality of different subcarriers. In a OFDM symbol time interval, each user in the cell use an orthogonal frequency subchannel, therefore, the OFDMA system is free from intra-cell interference, which is an important characteristic of the OFDMA system. When different cells or users use the same frequency subchannel for transmitting messages in the same time interval, an inter-cell co-channel interference (also called inter-cell interference) occurs. Thus, the link quality of the cell edge user degrades and the data throughput decreases, which are serious problems in an OFDMA system.
According to the IMT-Advanced/4G technical requirements issued by the International Telecommunication Union (ITU), the cell edge data rate has become an important performance index. In the future, the 4G mobile communication systems, such as the IEEE 802.16m, 3GPP LTE-Advanced, and 3GPP2 UMB+, all utilize inter-cell interference coordination technology to solve the problem of inter-cell interference. The inter-cell interference coordination technology is configured to coordinate the frequency, time, and/or emitting power between the neighboring cells in advance, in order to avoid or decrease inter-cell interference. Presently, various methods are used to implement inter-cell interference coordination. The most widely used methods are fractional frequency reuse and soft frequency reuse. Pre-4G technologies, such as 3GPP2 UMB and Mobile WiMAX (IEEE 802.16e), apply FFR technology against the problem of inter-cell interference. However, 3GPP LTE is developed to utilize soft frequency reuse technology to address the issue of interference.
FFR technology is a kind of frequency-domain interference coordination technology, which applies a frequency reuse factor (FRF) greater than 1 for planning the frequency in the cell edge region to reduce inter-cell interference. Thus the link quality is considered to be improved and the data throughput is considered to be increased. On the other hand, FFR technology applies an FRF equal to 1 (reuse-1 or FRF=1) in the cell center region to maintain superior system capacity.
FIG. 1 shows frequency resources allocation diagram for realizing partial frequency reuse in a mobile communication system. In the diagram, all useful frequencies are divided into center subband F1 12 and edge subband F3 11, wherein the edge subband F3 11 are further divided into three orthogonal subbands F3A, F3B, and F3C. Therefore, four orthogonal subbands are obtained. Referring to FIG. 1 the center subband F1 12 is adapted for the FRR equal to 1 (reuse-1) reuse method, which means all cells can use the subband. The edge subband F3 11 is adapted for the FRF equal to 3 (reuse-3 or FRF=3) reuse method, and the three subbands F3A, F3B, and F3C are adapted for the cell A13, cell B14 and cell C15 of the base station 1 respectively. Taking cell A 13 as an example, the center subband F1 12 of the cell A 13 is allocated to the neighbor users around the center of the cell, and the spectral efficiency of the subband is the highest. On the other hand, the edge subband F3A of the cell A 13 is first allocated to the cell edge user. At this moment, the reuse-3 reuse method is applied, and the link quality of the edge user is improved.
Soft frequency reuse is a broadcasting power interference coordination technology. The main concept is to broadcast more power to the cell edge area and less power to the center cell area. Since every cell can use the entire subband, soft frequency reuse is a technology of reuse-1 or FRF=1.
FIG. 2 shows the broadcasting power allocation for a base station using current mobile telecommunication soft frequency reuse technology. All available subbands are divided into center subband 22 and edge subband 21; and the edge subbands of cell A 23 and cell B 24 are orthogonal to the edge subband 21 of cell C 25. Pedge                and Pcenter represent cell edge subband power and cell center subband power, where the ratio of Pedge/Pcenter is greater than 1.        
Take cell A 23 for example, its center subbands (F2+F3) will be allocated for center users. Since the user is close to base station 1, there is less interference and less broadcasting power so as to provide sufficient signal quality while its edge subband (F1) will give priority to cell edge users. Because the broadcasting power of the edge subband of sector A 23 is stronger than the center subband broadcasting power of neighboring cells (Cell B 24 and Cell C 25), this can enhance the signal and reduce the interference and improve the connection.
So as to know, whether an OFDMA system uses partial frequency reuse or soft frequency reuse, cell users must be grouped into either center users or edge users. The traditional grouping method used in inter-cell interference coordination uses the user's geometry factor (GF). Geometry factor is defined as wideband average signal to interference plus noise ratio (SINR) as in Equation 1 below where P means broadcasting power, L means path loss, S means shadow fading, A means antenna gain, N means noise power. Sub-notation s and i represent server cell (SC) and interfering cell respectively. φ means the union of interfering cells.
                              G          ⁢                                          ⁢          F                =                                            P              s                        ·                          L              s                        ·                          S              s                        ·                          A              s                                                                          ∑                                  i                  ∈                  φ                                            ⁢                                                P                  i                                ·                                  L                  i                                ·                                  S                  i                                ·                                  A                  i                                                      +            N                                              Equation        ⁢                                  ⁢        1            
The acknowledged concept of user grouping in inter-cell interference coordination is:                1. First the user equipment (UE) measures the wideband average SINR of the serving cell or the serving sector and responds to the serving cell.        2. Then the serving cell determines the UE as either a cell center user or cell edge user. If the wideband average SINR is greater than the threshold level, then this UE is a cell center user; if it is smaller than it is a cell edge user.        3. When an UE gains top priority in the scheduler, the system will allocate a frequency subchannel from the center subband of the serving cell if it is a cell center user; and frequency subchannel from the edge subband of the serving cell if it is a cell edge user.        
Because using wideband average SINR to group users requires setting the wideband average SINR threshold level in advance and this threshold level depends on the geometry conditions such as inter-site distance. If the threshold level is not set in advance it will affect the ratio of cell center users to cell edge users and result in unpredictable system efficiency and complicate system usage. Besides, using wideband average SINR to group users will cause users with weak signals (i.e. wideband average SINR far below threshold level) to be prevented from being handed over to use cell edge subband. The system capacity will therefore be reduced if this kind of user increases and occupies the cell edge subband.